A "nest of spies" was quietly asked to leave Australia without charges laid after being uncovered by the nation's domestic spy agency.
ASIO director-general Mike Burgess has revealed more detail about the foreign agents which he last week said groomed politicians and sought access to security information for a major airport through a public servant.
He explained why the activities didn't lead to criminal charges.
"That example I gave of the 'nest of spies' as I called it, we got them early," he told a Senate estimates hearing in Canberra on Monday.
"It was actually deemed to be more appropriate we would ask those foreign agents to leave our country quietly and we did."
Independent senator Rex Patrick questioned what the phrase "nest of spies" indicated about the size of the group.
"You have to allow me some poetic licence, senator. That was just to make the point it was more than one," Mr Burgess said.
Mr Burgess said the levels of espionage and foreign interference in Australia were unacceptably high with agencies foiling a double-digit number of activities over the past year.
